The ongoing development of cars with respect to driving safety and increased requirements with respect to entertainment and infotainment results in a drastical increase of electronic modules in the car. Most of the electronic modules are integrated on a chip, wherein each electronic module includes a plurality of different functions, each integrated on one chip. Such electronic modules including different functions on one chip are micro controller units, called MCU. A plurality of safety-relevant applications in the automotive area, like airbags, ABS or the like require a reliable operation.
The reliable operation of MCUs and peripheral devices connected to MCUs strongly depends on valid clock signals. The reliable operation of the MCU may also be influenced by diagnostic units included or connected to the MCU. All of them, the MCU, the peripheral devices and the diagnostic units need to be supplied with reliable and stable clock signals. Clock faults like a missing clock or a deviation of the clock frequency can be critical for the operation of MCUs, especially in safety relevant applications. Such clock faults may for example prevent the execution of a safety function or may deactivate online-diagnostics.
When detecting a clock fault, an appropriate reaction must be initiated, which depends on the application requirements. In case of a critical clock failure it may be required to bring the whole system into a safe state. Conventionally all clocks used within the MCU are directly or indirectly derived from a system clock, which is used as reference for the detection of missing clock faults and for detecting frequency deviations by calculating the ratio between the frequency of such a derived clock within the MCU and the system clock frequency. In case that the ratio misses a predetermined value, a clock fault is detected. In other words, the correct detection of a clock fault relies on the presence of a correct system clock.